This is a National Stage entry under 35 U.S.C. xc2xa7371 of PCT Application No. PCT/JP01/00514 filed Jan. 26, 2001; the noted application is all hereby incorporated by reference.
The present invention relates to a molded article of a thermoplastic composite material and a method of producing the same.
Many attempts have been made so far to obtain molded articles having a wood-like texture by extrusion of a thermoplastic composite material comprising a mixture of a thermoplastic resin and a vegetable filler such as woodflour.
Japanese Kokai Publication Hei-07-266313 discloses a wood-like synthetic board obtainable by extrusion of a wood-like synthetic powder prepared by blending 20 to 75 weight % of a fragmented cellulosic material and 80 to 25 weight % of a thermoplastic resin molding compound and subjecting the blend to gelation-kneading and granulation.
However, a molded article of a thermoplastic composite material as prepared by increasing the amount of loading with a vegetable filler with over-emphasis placed on a wood-like texture, for example at a high filling rate of about 75 weight % based on the total weight of the thermoplastic composite material, has much to be desired in durability so that it is inadequate for use as an architectural material in the field. When such a molded article is subjected to an outdoor exposure test, discoloration and fading are observed in a short period of time, apparently due to release of the trace colored components occurring in the vegetable filler, such as lignin and hemicellulose.
Japanese Kokai Publication Hei-02-265731 discloses an extrusion method for a thermoplastic composite material containing a thermoplastic resin and a vegetable filler which comprises shaping the thermoplastic composite material in heated and molten state within an extruder equipped with a hot shaping die to a predetermined geometry and sizing the shaped material with a molding die disposed in contact with said hot shaping die and preheated to 80 to 140xc2x0 C.
However, in such an extrusion method, increasing the speed of extrusion of the thermoplastic composite material leads to an increased warp in the thickness direction of the molded article due to an uneven distribution of temperature. For this reason, there occurs the phenomenon that while the surface of the molded article is cooled and solidified, the core portion remains unsolidified so that only the core portion is extruded. Therefore, the speed of extrusion of the thermoplastic composite material cannot be increased beyond a certain limit, thus leading to low productivity.
Japanese Kokai Publication Hei-08-118452 proposes an extrusion method for a composite material in which a vegetable filler, such as woodflour, is filled into a thermoplastic resin, wherein the extrusion is effected under application of a restrictive force acting against the extrusion force at the metal die outlet to prevent formation of air bubbles and voids. According to this extrusion technology, however, the molding speed cannot be increased so that the productivity necessary for commercial production cannot be attained.
Such thermoplastic composite materials are characterized by the warm feeling inherent in the wood-like component. Attempts have also been made to create wood-like surface by surface cutting. In many of such instances, woodflour is used as the vegetable filler.
However, when woodflour is used as the filler, the molded article may undergo browning due to the decomposition of woodflour by heat to which it is exposed in the kneading with the resin component with the result that large amounts of colored pigments are required for colorizing the article to the desired color; the production apparatus is damaged by the pyrolysate of woodflour formed in the course of kneading; and the weather resistance of the article in outdoor applications is decreased, to mention a few of the consequent disadvantages.
The most striking of these disadvantages is the low weather resistance in outdoor applications. When the weather resistance is low, the inherent brown shade of woodflour is whitened in accordance with aging of weather resistance. The reason is decomposition of the lignin contained in the wood material.
To overcome this disadvantage, a technology inclusive of the method comprising causing titanium oxide particles to be immobilized on the surface of woodflour has been proposed (Japanese Kokai Publication Hei-06-39893). However, such measures are not effective enough to impart sufficient weather resistance.
Meanwhile, the technology using the material available on removal of coloring component, such as lignin, from a wood material, such as peroxide-decolorized woodflour or cellulose powder, as a filler has also been proposed. (Japanese Kokoku Publication Hei-02-33744, Japanese Kokoku Publication Hei-04-64337, Japanese Kokoku Publication Hei-04-7282, Japanese Kokai Publication Hei-10-245461, Japanese Kokai Publication Hei-10-316817, etc.)
These proposed technologies are capable of avoiding the browning of molded articles and the trouble of damage to production apparatus, which are problems caused by usage of woodflour, but since such fillers as cellulose powders are expensive materials as compared with woodflour, the problem of high material cost develops anew.
Japanese Kokoku Publication Hei-04-27924 discloses a molded article obtainable by sanding a molded article composed of a resin and a finely-divided cellulosic aggregate to give scratch marks oriented in one direction and thereby remove the residual internal stress of the surface skin layer and, then, embossing the surface to a wood grain pattern.
Several attempts have also been made to obtain thermoplastic composite materials by blending woodflour with thermoplastic resins such as PE and PP, and what are particularly needed are articles with wood grain motifs. For example, Japanese Kokai Publication Hei-08-207022 discloses an extrusion method of a resin material into a tubular form by means of an extruder using a circular die equipped with a resistance member having a multiplicity of fins or projections protruding inwardly from at least one of the outer peripheral surface side and the inner peripheral surface side, in an intermediate portion along the extrusion direction to impart a wood grain pattern by taking advantage of the extending and orienting effect due to said fins or projections on the colorant materials contained in the thermoplastic composite material.
In the former method among the above, the peripheral surface of the sanding roll to be used is provided with sanding edges arranged in a random pattern but since only the areas contacting the roll are selectively scraped, the difference in surface gloss from the adjacent surface skin layer is objectionably prominent. Moreover, the resulting scratch marks are rather suggestive of xe2x80x9cdamagesxe2x80x9d caused by the projecting metallic objects and may constitute a drawback of the molded article in the sense of the consequent risks for invasion of moisture in long-term use. Thus, because the above scratch mark is intended to remove the residual internal stress of the surface skin layer, the central zone of the mark forms a deep groove which is susceptible to moisture invasion and pooling.
In the latter method, the resin material once diverted by the resistance member having fins or projections joins again and becomes fused in the resin passage within the die but the zone of this fusion forms the so-called weldline which is inferior to the remaining part in mechanical properties. Furthermore, because of the existence of said weldline in the same layer of the same kind, a structure liable to undergo crack-propagative destruction tends to develop. Moreover, as the pattern imparted by said resistance member is subjected to heat in the downstream resin passage within the metal die, the colorant particles are dispersed to reduce the contrast, although the weldline is not abolished, thus making pattern control difficult. In addition, because said resistance member is disposed internally of the metal die, much labor is required for changing it to a different resistance member for imparting a different pattern.
In view of the above state of the art, the present invention has for its object to provide a molded article of a thermoplastic composite material which is not only durable with little discoloration or fading even on prolonged outdoor exposure and is very satisfactory in physical properties and appearance qualities inclusive of wood-like texture but also has high productivity and good economics.
The present invention, therefore, relates to a method of producing a molded article of a thermoplastic composite material comprising
extruding the thermoplastic composite material containing a thermoplastic resin and a vegetable filler,
the vegetable filler content being 50 to 90 weight % of the total weight of the thermoplastic resin and vegetable filler,
wherein extrusion is carried out by means of an extrusion apparatus comprising at least an extruder, a hot shaping die, and a cooling die,
the hot shaping die and cooling die being directly coupled.
In the method of producing a molded article of a thermoplastic composite material according to the invention, it is preferable that a coating layer composed of a resin material containing a thermoplastic resin is formed on the surface of a core layer obtainable by extrusion of the thermoplastic composite material.
The extrusion apparatus to be used in the method of producing a molded article of a thermoplastic composite material according to the invention preferably comprises the extruder, the hot shaping die, the cooling die, a surface heating die, and a hot coating die as serially arranged in this order, the hot shaping die and cooling die and the surface heating die and hot coating die being directly coupled respectively and, more preferably, the cooling die and surface heating die being also directly coupled.
It is preferable that primary cooling at 150 to 180xc2x0 C. is followed by secondary cooling at 60 to 140xc2x0 C. in said cooling die.
Preferably said extruder is a counterrotating twin-screw extruder and the temperature of the thermoplastic composite material at the outlet of the counterrotating twin-screw extruder is controlled within the range of (melting point xe2x88x9215xc2x0 C.) to (melting point +10xc2x0 C.) of the thermoplastic resin contained in the thermoplastic composite material.
Preferably said hot coating die has a shaping passage for the resin material and a supply passage for supplying said resin material to said shaping passage, said supply passage and said shaping passage intersecting each other, and said supply passage being internally provided with a projection protruding in the thickness direction of the passage.
Preferably said supply passage is internally provided with a groove.
A molded article of a thermoplastic composite material obtainable by said method of producing a molded article of a thermoplastic composite material constitutes a further aspect of the present invention.
Referring to said molded article of a thermoplastic composite material, said resin material preferably comprises at least two kinds of thermoplastic resins differing in melting point and being incompatible with each other.
Preferably the thermoplastic resin contained in said thermoplastic composite material is a polyolefin resin.
Preferably said coating layer is 0.1 to 0.5 mm in thickness.
Preferably said molded article of a thermoplastic composite material is a molded article having a profile.
Preferably the thermoplastic resin contained in said thermoplastic composite material is identical to the thermoplastic resin contained in said resin material.
A molded article of a thermoplastic composite material obtainable by said method of producing a molded article of a thermoplastic composite material wherein the resin material preferably contains a cellulose powder.
Preferably said cellulose powder content is 20 to 60 weight % of the resin material.
Preferably said coating layer is 0.1 to 3 mm in thickness.